Structures of the Proximal Forelimb and Shoulder

Scapula

The scapular cartilage is reasonably extensive compared to other species.

supraglenoid tubecle large and separate from glenoid cavity

Humerus

Greater tubercle have a cranial & caudal part

Intermediate tubercle

lesser tubercle similar size

Double intertubercular groove.

Radius and Ulna

The two bones are fused, so all movement is in a sagittal plane with very little /if any pronation and supination.

Radius is weight bearing

The ulna tapers out distally at the middle of the radius shaft.

For the proximal quarter of the radius the bones are joined by ligaments, then there is an interosseous space for the passage of blood vessels, distal to this the bones are fused.

The lateral styloid process of the radius is in fact developmentally the distal end of the ulna. It appears as a seperate ossification centre in a foal until it fuses at about a year old.

Joints of the Proximal Forelimb

Shoulder Joint

The joint capsule attaches a very short distance from the periphery of the articular surfaces.

The intertubercular (bicipital) bursa lies between the humeral tubercles cushioning the bicipital tendon.

The bursa and tendon are held in place by the transverse humeral retinaculum running between the greater and lesser tubercles of the humerus.

Elbow Joint

The joint capsule attaches to the articular surface of the condyle, the periphery of the olecranon fossa and the articular cartilage of the trochlear notch of the ulna. It fuses with the collateral ligaments.

Paired collateral ligaments attach the epicondyles to the tuberosities of the radius and ulna.

Joints of the Distal Forelimb

Carpal Joint

The antebrachiocarpal joint between the radius/ulna and the proximal carpal bones – most movement

The middle carpal joint between the two rows of carpal bones

The carpometacarpal joint between the distal carpal bones and the proximal metacarpals- least movement

The joint is a synovial joint, comprised of a common outer fibrous capsule and three inner synovial pouches, one for each joint.

Collateral ligaments extend from the radius to the metacarpal bones on the medial and lateral aspect of the carpus.

The carpal canal houses both the superficial and deep digital flexor tendon within a common synovial sheath.

Muscles of the Forelimb

Extrinsic Musculature

These muscle are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs as well as to stabilize the scapula.

Trapezius: Accessory n.

Origin: mid-dorsal raphe and supraspinous ligament

Insertion: spine of the scapula

Body: two parts, cervical and thoracic separated by aponeurosis

Action: raises scapula against the trunk and swings cranially to advance the limb

Passes through carpal canal before branching and continues to palmar distal phalanx

Interosseus muscles/Suspensory ligament

Stay Apparatus of the Forelimb

When a horse is standing muscles on opposite sides of joints delicately contract and relax so that it stays in balance. This obviously buts considerable strain and stress on the muscles. To combat this the horse has increased the fibrous content of many muscles. These collagenous muscles have been named collectively as the stay apparatus. This is further divided into the suspensory apparatus and the check apparatus.

Suspensory Apparatus

Interosseous medius/Suspensory ligament

Proximally this attaches to the distal row of carpal bones and metacarpus. It runs distally on the palmar surface of the metacarpal bone and then bifurcates proximal to the fetlock, with a small branch connecting to each sesamoid.

It then continues distally on the dorsal surface of the proximal phalanx and joins the common digital extensor, which continues to insert on the middle and distal phalanges.

Associated ligaments

Collateral sesamoid ligaments – connect the abaxial surface of the sesamoid to the metacarpus and proximal phalanx.

Palmar ligaments – connect the sesamoid bones together.

Distal sesamoidean ligaments – connect the sesamoids to the proximal and middle phalanges.

This ligaments can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.

Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the fetlock and pastern through the distal sesamoidean ligaments and resist flexion of the pastern and coffin joints through the common digital extensor tendon.

Stay Apparatus

Serratus ventralis support the cranial part of the body and connect the costal side of the scapula to points of attachment on the caudal caervical vertebrae and cranial ribs. It contains a tendinous layer that suspends the body once the muscle relaxs. This though will cause flexion of the shoulder which needs to be balanced by the extensors of the shoulder.
The Biceps brachii fulfils this role. It’s collagenous tendon extends the length of the muscle and divides near the elbow. The short tendon inserts on the radial tuberosity, the long tendon (lacertus fibrosus) continues distally and blends with the tendon of the extensor carpi radialis and forearm fascia; combined these insert on the proximal end of the metacarpus.
Thus the biceps has provision to be able to relax without the collapse of the shoulder, elbow and carpal joints.

Check Apparatus

The superficial digital flexor muscle continues past the carpus as a thick tendon. The accessory or superior check ligament is it’s tendinous radial head that joins the muscle at this point. The tendons continue distally and bifurcate by the fetlock into slips that attach to the medial and lateral aspect of the pastern joint. There is therefore a ligamentous connection from the distal radius to the pastern that can help prevent extension of the carpal and fetlock joint without muscle contractions.

The deep digital flexor also forms a thick tendon just proximal to the carpus. The accessory ligament or inferior check ligament joins the tendon at the middle of the metacarpus. They then continue through the bifurcation of the superficial flexor tendon, through the intersesamoidean groove at the fetlock joint, over the pastern and inserts on the distal phalanx. This ligamentous tissue between the metacarpal region and distal phalanx helps prevent extension of the fetlock, pastern and coffin joints.

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About Annettevet

I'm a Norwegian practicing as a veterinarian in Durham, England. I`ve recently qualified as a veterinarian at Glasgow University. This is a blog about my experiences as a vet student and now as a veterinarian.
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